Evaluating stability of a Raman spectrometer for long-time experiments

Raman spectroscopic measurements spanning several hours to days are increasingly common and important. The stability of a Raman spectrometer becomes crucial when comparing spectra recorded at different occasions in such time-intensive experiments. In this work, series of Raman spectra of benzene and emission spectra of neon were acquired using a standard Raman spectrometer operating in a typical air-conditioned laboratory, with synchronized measurements of ambient parameters (pressure, temperature, humidity, etc.) subject to continuous unsystematic environmental drifts. The time-dependent fluctuations of positions and intensities of selected bands in the series of spectra recorded over tens of hours displayed variations of up to ±0.3 cm⁻¹ in the band positions and up to 2% in the associated Raman intensities, with experiments over longer time periods prone to more pronounced fluctuations. After accounting for change in the laser power fluctuations, the intensity variations were reduced to about 1%. Both the variations in the band positions and intensities showed practically no correlation with either of the ambient parameters or the refractive index of air. Tiny drifts in the laser wavenumber (on the order of 0.01 cm⁻¹), attributed to the change in the refractive index of the laser cavity associated with the variations in the ambient pressure, were too small to account for the observed fluctuations in the band positions.